I believe you should be there for your friends.
I would sit down with them both, tell them you know where they are, how they feel and ask how you can help.
I would let your friend know you intend on supporting her BS and I would do just that.
Caution: opposite sex BS comfort in real life is dangerous. I think quick but shallow emotional bonding would stand a high chance if occurring
I don't see this as an issue if you are both clear on boundaries. I have supported a male BS before, I would never take advantage of their pain.
I would also share SI with the BS first, let him know what it is and how it has helped you.
I don't think I would share my user name with him though, IMO that's too close for comfort
Ultimately you give what you can without having any negative impact on you.